In most cases this is the code you need to choose while adding HTTP check. It means everything goes as it should. If you are not sure which code to pick up take this one.
301 Moved Permanently
This code means that this and all future requests should be directed to the given URI. Sometimes you may want to redirect user to some another page and your web server (nginx or Apache) responses with this code. In most cases it's okay.
This one is more tricky. Old title for it was "Moved Temporarily" and now applications often use it for similar to code 301 purposes, i.e. to redirect user. You may want check mentioned above sources to get more information.
4xx Client Error
This code is used by web server to tell browser to provide authorization data, i.e. login and password. It is being used for Basic and Digest access authentications.
This response means you don't have permission to access the resource.
5xx Server Error
500 Internal Server Error
A generic error message. Often it means that error has happened within application, for instance, fatal PHP errors or an error because MySQL inaccessibility.
502 Bad Gateway
We bet you have already seen this error code. It could happen, for instance, if you use nginx and php-fpm and for some reasons php-fpm doesn't work.
504 Gateway Timeout
HTTP code 504 means that web server (say, nginx) couldn't get timely response from upstream (in most cases, it's PHP).
749 Reserved for Chuck Norris
Nothing to add.